1. Field of the Invention
The present invention relates to an optical pickup apparatus in an optical information recording/reproducing apparatus which reads and write signals from and onto an optical disk as an optical information recording medium.
2. Description of the Related Art
The numerical aperture of the objective lens in an optical pickup apparatus may be increased in order to improve the recording density of an optical disk. For example, the numerical aperture is increased to 0.6 from 0.45. According to the standards that have been defined to record information from the information recording surface through a transparent disk substrate having a predetermined thickness, comatic aberration occurs then depending on the inclination of the transparent disk substrate so that the adequate beam spot cannot be irradiated on the information recording surface. As the numerical aperture increases, the amount of the comatic aberration occurred increases prominently. When optical disks are prepared at a low cost using transparent disk substrates which have been produced by injection molding or the like, particularly, the inclination of the transparent disk substrate caused by the warping of the optical disk gets greater. This makes the adverse influence of the comatic aberration greater.
To reduce the influence of the comatic aberration, the transparent disk substrate is made thinner. For instance, the substrate thickness is reduced to 0.6 mm from 1.2 mm. This causes stain, scratches or the like on the surface of the optical disk to noticeably affect information recording/reproduction, thus deteriorating the performance of the optical disk over a long period of usage.
Another solution is to install a tilt (skew) adjusting mechanism for an optical disk into a pickup apparatus and use a tilt servo mechanism which tilts the pickup itself in responsive to the tilt of the optical disk. This scheme suffers a slow response speed due to the large inertial weight of the movable portion. The tilt of the pickup itself cannot therefore follow up a variation in the tilt of the optical disk during one rotation, particularly, the tilt in the direction of the time axis (tangential direction). Accordingly, correcting the comatic aberration caused by the tilt in the tangential direction raises a problem in a pickup apparatus which uses an objective lens having an increased numerical aperture.
One known scheme of correcting the comatic aberration is disclosed in Japanese Unexamined Patent Publication No. Hei 7-140381. As shown in FIG. 1, the comatic aberration is corrected by mutually moving two compensators, which are the combination of a convex surface whose curvature changes by an inclination of R to the fourth power where R is the radius of a pupil, in a direction orthogonal to the optical axis and a concave surface which matches with the convex surface.
If an optical disk 5 is tilted in the tangential direction, aberration in the tangential direction is produced. When a convex lens having a relatively large numerical aperture, such as an objective lens 4, is used, the aberration of the third order or the so-called comatic aberration is large in addition to the astigmatism and high-order aberration. The greater the numerical aperture is, the more noticeable the high-order aberration becomes.
The curve (A) in FIG. 2 shows a change in the aberration-originated optical path difference within a pupil when the transparent disk substrate of the optical disk 5 tilts. The abscissa axis in the figure represents the radial position on the pupil and the ordinate axis represents the produced phase difference of the wavefront as the optical path difference with one wavelength as a unit. In the idealistic aberration-free state, the optical path difference does not occur because the light beam comes perpendicularly in the state so that the peripheral edge is symmetrical to the optical axis centered in the optical path. FIG. 2 shows the results of computing the optical path difference when the optical disk is tilted by an angle of 1 degree from the horizontal state with the numerical aperture of the objective lens being 0.85. This amount of change varies in accordance with the tilt amount of the optical disk.
This aberration is produced by the tilt of the transparent disk substrate of an optical disk, and the upper-leftward increase of the curve of the optical path difference shown in FIG. 2 indicates the deflection of light in the advancing direction from the wavefront or the optical axis which is caused by the tilted transparent disk substrate.
Correcting the wavefront aberration by inserting the compensators having the reverse wavefront aberration having the inverse shape to that of the wavefront aberration (curve (A) in FIG. 2) into the optical path that extends from, for example, the light source to the objective lens imparts deflection in the advancing direction of light in addition to the formation of the wavefront for forming a diffraction limited spot on the optical disk. As shown in FIG. 1, the deflection produces an adverse effect of causing the light beam to obliquely enter the objective lens, thus lowering the image forming performance. Further, when the compensators are used to correct the tangential tilt, the deflection causes the irradiation position to be shifted because an ordinary optical pickup apparatus has no means for adjusting the shift of the irradiation position in the tangential direction, and the positional shift results in jittering in the tangential direction. As the amount of aberration correction necessary at the peripheral portion is large, the positional-shift originated correction error that becomes severe in proportion to the amount of correction becomes larger. This makes it difficult to correct the comatic aberration by an increased numerical aperture.
That is, according to the conventional method, the optical axis is deflected by the movement of the inserted compensators, thus shifting the center position of the spot to be irradiated on an optical disk. This deflection of the irradiation optical axis acts as time-dependent noise (jitter) particularly in correcting the tilt in the tangential direction, so that the tangential tilt cannot be corrected adequately.
Accordingly, it is an object of the present invention to provide an optical pickup apparatus capable of preventing the deflection of the irradiation optical axis which is caused by movement of compensators in the conventional correction of the comatic aberration, thereby well removing the time-dependent noise component.
An aberration correcting device according to a first aspect of the present invention being placed in an optical path extending from a light source of an optical pickup apparatus for irradiating a light beam onto an optical disk having a light-permeable layer to the optical disk, comprises:
a pair of light-transmission substrates having complementary curved surfaces facing, and apart from, each other, at least one of the light-transmission substrates being held freely movable in such a way that a major surface thereof moves in a direction vertical to the optical path,
the complementary curved surfaces being formed so as to make a relative movement of the light-transmission substrates vary an optical path length of a light beam transmitting the light-transmission substrates to impart a phase difference to the transmitting light beam, while allowing the phase difference to keep an advancing direction of the light beam thereby to minimize a comatic aberration imparted to the transmitting light beam by the light-permeable layer of the optical disk.
In an embodiment of the aberration correcting device of the invention, the complementary curved surfaces are symmetrical to each other at least in a radial direction or tangential direction of the optical disk.
In an embodiment of the aberration correcting device of the invention, the complementary curved surfaces have shapes satisfying a following equation respectively:
z=(ax)4xe2x88x92(bx)2
where xe2x80x9czxe2x80x9d is a height in a direction parallel to the optical path and x is a radius about the optical path and 2a2=b.
In an embodiment of the aberration correcting device of the invention, one of the light-transmission substrates has a convex surface around the optical path and a normal line of the curved surface at an outermost portion within an effective radius is parallel to the optical path, and the other light-transmission substrate has a concave surface around the optical path and a normal line of the curved surface at the outermost portion within the effective radius is parallel to the optical path.
In an embodiment of the aberration correcting device of the invention, one of the light-transmission substrates has a convex surface around the optical path and a recess formed at an outer portion within an effective radius and a normal line of the curved surface at a bottom of the recess is parallel to the optical path, and the other light-transmission substrate has a concave surface around the optical path and a protrusion formed at an outer portion within the effective radius and a normal line of the curved surface at a top of the protrusion is parallel to the optical path.
In an embodiment of the aberration correcting device of the invention, one of the light-transmission substrates has a convex surface with a center being a symmetrical surface which is a plane including the optical path and a normal line of the curved surface at an outermost portion within an effective radius is parallel to the optical path, and the other light-transmission substrate has a concave surface with a center being the symmetrical surface and a normal line of the curved surface at the outermost portion within the effective radius is parallel to the optical path.
In an embodiment of the aberration correcting device of the invention, one of the light-transmission substrates has a convex surface with a center being a symmetrical surface which is a plane including the optical path and a recess formed at an outer portion within an effective radius and a normal line at a bottom of the recess is parallel to the optical path, and the other light-transmission substrate has a concave surface with a center being the symmetrical surface and a protrusion formed at an outer portion within the effective radius and a normal line at a base of the protrusion is parallel to the optical path.
In an embodiment of the aberration correcting device of the invention, at least one of the light-transmission substrates is driven within a cylindrical surface with one point on the optical path being a center.
In an embodiment of the aberration correcting device of the invention, at least one of the light-transmission substrates is driven within a spherical surface with one point on the optical path being a center.
An optical pickup apparatus, according to the invention, for reading signals from and/or writing signals onto an optical disk for reading signals from and/or writing signals onto an optical disk having a light-permeable layer, comprises:
a light source for emitting a light beam;
an objective lens for focusing the light beam onto an information recording surface of the optical disk;
an optical irradiation system for guiding the light beam to the objective lens along an optical path;
an optical detection system, including photodetection means, for guiding reflected light from the information recording surface to the photodetection means;
tilt detection means for detecting a tilt of the optical disk at least in a radial direction or tangential direction thereof;
aberration correcting means located in the optical irradiation system and comprising a pair of light-transmission substrates having complementary curved surfaces facing, and apart from, each other, at least one of the light-transmission substrates being held freely movable in a direction vertical to the optical path, the complementary curved surfaces being formed such aspherical surfaces as to make a relative movement of the light-transmission substrates vary an optical path length of a light beam transmitting the light-transmission substrates to impart a phase difference to the transmitting light beam, while allowing the phase difference to keep an advancing direction of the light beam; and
aberration correction driving means for moving the at least one of the light-transmission substrates in a direction vertical to the optical path in accordance with an output of the tilt detection means corresponding to a tilt amount of the optical disk thereby to minimize a comatic aberration at least in a radial direction or tangential direction which is imparted to the transmitting light beam by the light-permeable layer of the optical disk.
In an embodiment of the optical pickup apparatus of the invention, at least one of the light-transmission substrates is driven within a cylindrical surface with one point on the optical path being a center.
In an embodiment of the optical pickup apparatus of the invention, at least one of the light-transmission substrates is driven within a spherical surface with one point on the optical path being a center.
According to the aberration correcting device of the present invention for use in an optical pickup apparatus, two light-transmission substrates have complementary internal surfaces respectively. The light-transmission substrate have partially different complementary thicknesses respectively in a direction of the axis of irradiation light i.e., optical path and are arranged approximately in parallel to each other in such a way that their optical axes coincide with the irradiation light axis, and one of the light-transmission substrates is moved approximately vertical to the irradiation light axis so that the length of the optical path of the transmission light or the total thickness of the light-transmission substrates can be changed partially. This imparts a predetermined phase difference distribution to the transmission light beam to ensure fast wavefront correction with a smaller amount of correction. Further, it is possible to correct aberration without causing the shift of a beam spot caused by the deflection of the transmission light, so that a time axis variation-dependent noise component can be adequately removed. According to the optical pickup apparatus of the present invention, as apparent from the above, the partial length of the optical path of the transmission light beam is changed by moving the light-transmission substrates to suppress or eliminate deflection of the optical axis. This prevents the irradiation light axis from being deflected by the movement of compensators, so that the irradiation beam spot will not be shifted.